"i see a vacant seat," replied the ghost, "in the poor chimney corner, and a crutch without an owner, carefully preserved." what do these lines mean?

Those line mean that if Scrooge doesn't help the Cratchit family, the boy will probably die. Tiny Tim has to walk using crutches because he is suffering from a terrible disease (probably polio) and the Ghost says that if he doesn't get the medical help he needs, he will probably die and leave 'a crutch without an owner.'